1. Field of the Invention
The present invention relates to a bearing material for a hydrostatic gas bearing having a porous sintered metal layer and a hydrostatic gas bearing using this bearing material.
2. Description of the Related Art
Porous hydrostatic gas bearings have hitherto attracted attention as having excellent high-speed stability and high load carrying capacity, and although various studies have been made, there are yet a number of problems to be overcome in their practical use.
As for the porous hydrostatic gas bearing, a bearing material which is formed by combining a porous sintered metal compact with a backing metal provided with means for supplying a compressed gas is frequently used. As the material for forming the porous sintered metal compact in this bearing material, a material which mainly consists of bronze, an aluminum alloy, or stainless steel, particularly a material which mainly consists of bronze, is frequently used.
As the bearing material used for the porous hydrostatic gas bearing, sufficient gas permeability and the surface roughness on the order of 10−3 mm are required. However, in the case where the above-described bearing material is used for the hydrostatic gas bearing, the porous sintered metal compact itself has satisfactory gas permeability in a way, but since the dimensional accuracy and surface roughness of the porous sintered metal compact are not sufficient, its surface is subjected to machining in many cases.
This machining is mainly effected by lathe turning, milling, and/or grinding, but loading of the surface of the porous sintered metal compact is caused by such lathe turning, milling, and/or grinding, which substantially affects its gas permeability (drawing characteristic). In grinding, in particular, plastic flow takes place in the surface of the porous sintered metal compact, and burrs are consequently caused.
In addition, the porous sintered metal compact is combined with the backing metal provided with the means for supplying a compressed gas as described above, and in the case of, for example, a porous hydrostatic radial gas bearing, a means for press-fitting a hollow cylindrical porous sintered metal compact to a hollow cylindrical backing metal is adopted in this combining process.
In the case of a simple slide bearing, no particular problem occurs even if such a press-fitting means is adopted. In the porous hydrostatic gas bearing, however, since a very small gap is present between the contact portions of the two members which are apparently tightly press-fitted to each other, there are cases where the leakage of the gas from this gap is greater than the essential circulation of the compressed gas in the porous sintered metal compact. The leakage of the gas from this gap naturally leads to a decline in the performance such as a reduction of the load capacity as the porous hydrostatic gas bearing, so that it is preferable to prevent this leakage as much as possible.
To cope with this problem, if the interference is made large and fitting is effected with a large press-fitting force, the gap in this portion can be eliminated substantially completely. On the other hand, however, there is the possibility of occurrence of plastic flow of the sintered metal on the outer surface side of the porous sintered metal compact subjected to extremely large drawing by the backing metal. Hence, a problem newly arises in that, after fitting to the backing metal, the circulation of the compressed gas is substantially hampered on the fitted surface side of the porous sintered metal compact.
In view of the above-described problems, the present assignee proposed a technique such as the one disclosed in JP-A-11-158511 (hereafter referred to as the conventional technique) to overcome the above-described problems. Namely, this conventional technique concerns a bearing material for a porous hydrostatic gas bearing includes: a backing metal; and a porous sintered metal layer sintered onto at least one surface of the backing metal, particles of an inorganic substance being contained at grain boundaries of the porous sintered metal layer. As a specific example, this conventional technique further discloses a porous sintered metal layer which is composed of, in addition to the particles of the inorganic substance, 4 to 10% tin, 10 to 40% nickel, 0.5 to 4% phosphorus, 3 to 10% graphite by weight, and the balance consisting of copper.
The bearing material disclosed in this conventional technique offers the following advantages: (1) Since particles of an inorganic substance such as graphite are contained at grain boundaries of the porous sintered metal layer, even if the bearing material is subjected to machining, the loading of its surface is suppressed, and an ideal drawn structure can be obtained. (2) Since the porous sintered metal layer is integrated with the backing metal by bonding, the leakage of a compressed gas from this junction is nil, and the deformation of the sintered layer due to the supplied gas pressure can be reduced to a minimum.
As for the porous sintered metal layer of the bearing material disclosed in this conventional technique, nickel (Ni) and phosphorus (P) among the components produce liquid-phase Ni3P in the sintering process, and the alloying of the sintered layer takes place by the mutual diffusion between the solid phase and the liquid phase, which becomes gradually active with a rise in the sintering temperature. In addition, the bearing material is fabricated through integration by bonding between the porous sintered metal layer and the backing metal with excellent wettability of the liquid-phase Ni3P with respect to the backing metal (steel product).
However, in a case where stainless steel excelling in corrosion resistance, particularly rust resistance, is used as the backing metal, a number of problems were presented in the bonding and integration of the backing metal and the porous sintered metal layer. Namely, these problems include: (1) In a case where the porous sintered metal layer is bonded to at least one surface of the backing metal consisting of stainless steel at the time of sintering, chromium oxides such as Cr2 O3 are formed on the surface of the backing metal, i.e., at the bonded interface between the backing metal and the porous sintered metal layer. Since the chromium oxides are interposed at the bonded interface, the bonding and integration of the porous sintered metal layer onto the backing metal surface is hampered. (2) If the amount of liquid-phase Ni3P produced during sintering is large, such liquid-phase Ni3P flows out during sintering, and the liquid-phase amount of Ni3P necessary for bonding the porous sintered metal layer to the backing metal surface decreases, thereby weakening the bonding strength between the porous sintered metal layer and the backing metal. Thus the porous sintered metal layer shrinks at the junction between the porous sintered metal layer and the backing metal along with a decline in the temperature during cooling (radiational cooling) after sintering, resulting in an exfoliation at the junction. In particular, the above-described problem (2) brings about drawbacks such as the leakage of a compressed gas from the junction in the porous hydrostatic gas bearing.
As a result of conducting research in view of the above-described problems, the present inventors discovered that, with respect to the above-described problem (1), if a plating layer is provided on the surface of the backing metal consisting of stainless steel, and the bonding layer consisting of such a plating layer is interposed between the backing metal and the porous sintered metal layer, it is possible to prevent the formation of chromium oxides at the bonded interface between the backing metal and the porous sintered metal layer, and the porous sintered metal layer can be bonded to and integrated with the surface of the backing metal consisting of stainless steel by means of the bonding layer. In addition, the present inventors discovered that, with respect to the above-described problem (2), the amount of shrinkage of the porous sintered metal layer during cooling after sintering can be reduced by decreasing the amount of liquid-phase Ni3P produced, and that it is therefore possible to effect the bonding and integration without causing exfoliation at the junction between the porous sintered metal layer and the backing metal, and enhance the porosity of the porous sintered metal layer to increase the amount of flotation by the compressed gas circulating through the porous sintered metal layer.